1. Field of the Invention
The present invention generally relates inspection systems for inspecting leads on circuit packages and, more particularly, to inspection systems that inspect the leads on circuit packages of the surface-mounted type.
2. State of the Art
Circuit packages of the surface-mounted type are designed, as their name implies, for electrical connection to the surface of a circuit board. Typically, leads on surface-mounted circuit packages are finer and more closely spaced than leads on circuit packages of the dual in-line pin (DIP) type. For mounting surface-mounted packages to a circuit board, solder paste is silk-screened onto minute solder pads that are precisely located on the board to match the pattern, or "footprint", of properly aligned leads extending from the circuit package. After the desired number of circuit packages are placed on a board with the package leads embedded in the solder paste on the pads, the packages are permanently soldered into place. To achieve satisfactory electrical connections using surface-mounted packages, not only must the lead patterns of the packages match the patterns of pads on a circuit board but, also, all of the leads of the circuit package must make contact with the intended solder pads. This latter condition is often expressed by saying the leads of surface-mounted packages must be substantially coplanar. Typically, surface-packages are required to have the "contact ends" of their leads coplanar within less than about two to about four thousandths of an inch (mils).
As used above, the term "contact end" refers to the portion of a package lead which is intended to contact a solder pad when the package is placed in its normal position on a circuit board. Equivalently, the term "contact end" can be understood to refer to the portion of a circuit package lead which is closest to a flat reference surface when the circuit package rests in its normal position on the reference surface. It should be noted that the actual end of a lead of a surface-mounted package can be significantly different from the contact end, since surface-mounted circuit packages can have leads of various shapes including "J" and gull-wing like shapes. It should also be noted that, when a circuit package is viewed from its side, the contact end of a lead appears to have an edge; such an edge can be referred to as an "apparent edge".
Currently, several methods are used for detecting whether the contact ends of leads on surface-mounted circuit packages are coplanar. The conventional detection methods include reflected image comparisons, image analysis employing video cameras, transmitted light measurements, laser-based dimensional measurements, and cast-shadow analysis. Each of these inspection methods is discussed in copending U.S. patent application Ser. No. 153,764, filed Feb. 8, 1987, the disclosure of which is herein incorporated by reference. As set forth in the application, each of the known inspection methods has substantial drawbacks.
In particular it should be mentioned that workers in the art have used matrix-type video cameras in systems that inspect the leads of circuit packages. In this context, the term matrix-type video camera refers to video cameras that depict scenes in two dimensions. The use of matrix-type video cameras in systems for inspecting rows of pins on integrated circuit packages of the dual in-pin (DIP) type is described, for example, in U.S. Pat. No. 4,696,047. In the patented inspection system, a conveyor moves DIP packages past a matrix-type video camera which provides two-dimensional images of the pins. The images are analyzed by a signal processor which compares the images with references stored in the memory of a digital computer. Based upon the image comparisons, the inspection system rejects circuit packages whose pins do not meet specified tolerances.
Also, U.S. Pat. No. 4,668,939 discloses an automated system using a matrix-type video camera for inspecting solder bumps on chip carriers. According to the patent, the images provided by the matrix-type video camera are processed to provide one-dimensional intensity plots. The patent states that the intensity plots can be analyzed to detect missing, bridged or excessive solder bumps.
Image analysis employing matrix-type video cameras has several disadvantages. One difficulty arises from the fact that the two-dimensional video information is normally arranged in rectangular rasters having aspect ratios of about 4:3. By way of contrast, the region of interest when inspecting leads of circuit packages of the surface-mounted type can be, for example, about two-thousand wide by twenty mils, which equates to an aspect ratio of about 100:1. (In this example, the two-thousand mil dimension would represent the length of a surface-mounted package and the twenty mil dimension would encompass a space between the contact end of a lead and a reference surface.)
To increase aspect ratios when inspecting leads on circuit packages of the surface-mounted type with matrix-type video cameras, it is known to fit the cameras with cylindrical lenses that provide image magnification in the vertical direction greater than in the horizontal direction. That is, such lenses increase resolution in the vertical direction while retaining sufficient horizontal field width to allow viewing of an entire side of a circuit package. Even with such cylindrical lenses, however, matrix-type video cameras may not provide adequate resolution in both the vertical and horizontal dimensions for inspection of leads of circuit packages of the surface-mounted type.
At this juncture, it can be mentioned that linescan camera comprising linear arrays of photosensitive elements are also known. For example, a linescan camera comprised of photodiodes is disclosed, in U.S. Pat. No. 3,781,467. According to this patent, the linescan camera operates with a microscope to measure the length of non-metallic inclusions in polished steel specimens. Also, a linescan camera is disclosed in U.S. Pat. No. 4,520,702 which is directed to an apparatus for inspecting and cutting articles.
In a somewhat different technique for inspecting leads on surface-mounted circuit packages, fiber-optic sensors have ben used to make so-called "transmitted light" measurements. According to this technique, when a circuit package is placed on its leads on a planar surface, any gap between the contact end of a lead and the planar surface can be considered to be analogous to a gate. The quantity of light transmitted through a given "gate" is directly proportional to the gap between the contact end of an inspected lead and the reference surface. By measuring the light intensity at the gate locations, one can determine the extend to which the contact ends of leads of a circuit package are coplanar.
The technique of transmitted light measurements also has several disadvantages when inspecting circuit packages of the surface-mounted type. One drawback is that a complete sensor head must be custom designed and fabricated for each package configuration. Another disadvantage is that conversion of such systems from one package type to another is difficult and time-consuming. Still another drawback is that the transmitted light measurements can vary depending upon whether the apparent edges at the contact ends of the leads are horizontal or angled from horizontal.
In view of the preceding, it can be understood that a need exists for improvements in systems and methods for inspecting leads on circuit packages of the surface-mounted type to determine the extent to which the contact ends of leads are coplanar. Also, a need exists to provide systems and methods for measuring geometrical properties of leads on surface-mounted circuit packages such as the width of leads, distances center-to-center between leads, and spacings edge-to-edge between leads.
As will be evident in view of the following description, one advantage of the present invention is that leads on circuit packages of the surface-mounted type can be rapidly inspected with very high resolution. Another advantage of the present invention is that circuit packages of the surface-mounted type having a variety of sizes, shapes and lead types can be inspected without substantial modifications to the measurement system and without substantial changes in measurement accuracy. These and other advantages of the present invention can be ascertained by reference to the following description and attached drawing which illustrate the preferred embodiment.